Fuels and fuel oils used in combustion machines such as gas turbines and diesel engines may contain impurities that cause or facilitate undesirable corrosion of the components of these systems. For example, due to the high temperatures in the “hot gas path” of a gas turbine (e.g. in the range of 580° C.-2000° C.), turbine blades and guide vanes are constructed of superalloy materials. The natural oxidation rate of these materials is relatively low which facilitates a significant component life, but if the oxidation rate is increased by the presence of corrosive deposits, then their life can be reduced to months. Sulfidation and vanadic corrosion are the two main types of accelerated high temperature corrosion mechanisms that can occur on gas turbine blades, both involving molten salt corrosion and ash melting point.
Sulfidation corrosion results from the presence of the alkali metals sodium and potassium in the presence of fuel sulfur. Vanadic corrosion is a generalized corrosion mechanism that results from the presence of vanadium in the fuel. During combustion, vanadium is oxidized to vanadium pentoxide, V2O5, which condenses on blade surfaces. Vanadium pentoxide reacts with the protective oxide film on turbine blades, which depletes the surface of its protective oxide layer. The cyclic replenishment of the oxide film on the blade surface results in a gradual “eating away” of the blade surface. Also, the severity of vanadic corrosion is significantly increased in the presence of sodium and potassium, as low melting point eutectic compounds can be formed even at 530° C.
In diesel engines utilizing contaminated fuels, high temperature corrosion of valve and valve seats combined with ash fouling of the hot gas path can cause major problems. Although vanadium pentoxide alone can be a severe problem in causing deposits/corrosion as it has a melting point within the operating temperature of diesel engines, sodium and sulfur compounds make this problem significantly worse. Deposits in a molten state will attack the metal components within a diesel engine leading to metal thinning and eventually failure of the component. Also, the build up of ash deposits in a diesel engine will impede the free flow of gases and reduce the efficiency of the turbocharger.
The use of a fuel oil additive containing magnesium in a highly reactive form allows for the safe and efficient operation of a gas turbine or diesel engine when utilizing a heavy fuel oil containing metal and sulfur contaminants. The magnesium compounds react with the vanadium, sulfur and other contaminants forming ternary compounds that are not detrimental to the system. Highly reactive magnesium compounds can counteract the effects of both ash deposition and high temperature corrosion by increasing both the fusion point of ash components that are formed and modifying the ash that does form to a soft, powdery and friable form. Such additives may be termed fireside additives because they are added to the fuel burned in the gas turbines or boilers to prevent high temperature corrosion and fouling.
During combustion of the fuel, magnesium compounds react with the vanadium oxides to form high melting point magnesium vanadates, which melt at temperatures well above those encountered in gas turbines and diesel engines. For diesel engine applications, the influence of sodium and potassium must be considered and, therefore, as the levels of sodium and potassium increase relative to vanadium, more magnesium may be required to counteract these resulting detrimental effects. Deposits that are no longer molten will not be corrosive.
It is known to add magnesium, as magnesium sulfate (epsom salt), magnesium acetate, magnesium chloride, magnesium oxide or magnesium carbonate to fuels for diesel engines and gas turbine containing vanadium to reduce the corrosion in the turbine blades. In particular, magnesium overbase additives are typically used to inhibit sulfidation and vanadic corrosion that may be caused by fuel oils containing these impurities. U.S. Pat. Nos. 4,163,728 and 4,179,383 describe stable, fluid magnesium-containing dispersions and preparations thereof by high temperature decomposition of magnesium salts of carboxylic acids to MgO in dispersant-containing fluids. However, many magnesium overbase additives are affected by the presence of water, and acidic or CO2 environments and may be unstable during storage and/or use.
There is thus a need to discover new magnesium overbase additives that have improved hydrolytic stability, and which still permit the magnesium overbase additives to function as corrosion inhibitors in fuel oils.